Battery Relocation Wire Size

Discussion pertaining to positive pressure E28s.
Post Reply
Dirtym30
Posts: 56
Joined: Jan 12, 2020 5:30 PM
Location: Toronto, Canada

Battery Relocation Wire Size

Post by Dirtym30 »

Hi All,

I'm currently in the final stages of the turbocharging process. I am curious what your thoughts are on what Gauge wire you would recommend when throwing the battery in the trunk.

I have some spare 1/0 gauge laying around I can use. Will this be ok?

I've searched the forums but nothing was really set in stone clear to me. Also, a bonus would what amperage circuit breaker would you adjust to govern the system.

Thanks in advance all. I hope all your winter projects are coming along in time for summer!

PFA
Image
Galahad
Posts: 372
Joined: Sep 06, 2016 9:08 PM
Location: Beverly/Worcester, MA
Contact:

Re: Battery Relocation Wire Size

Post by Galahad »

1/0 gauge will be fin - it'll do 150A continuous, way more than that for short periods.
for circuit breaker:
Starter is (overestimated) ~2hp = ~1500w. At 12V that's about 125A. I wouldn't go less than 150, probably not hugely more than that either. The problem with really low intentional resistance values is any short might have enough resistance to keep the breaker from blowing.
It'll be somewhat difficult to measure the actual starter current unless you get one of those current measuring clamp meters - if you have one of those I'd suggest measuring the actual value and sizing up from that a bit.
Dirtym30
Posts: 56
Joined: Jan 12, 2020 5:30 PM
Location: Toronto, Canada

Re: Battery Relocation Wire Size

Post by Dirtym30 »

Thanks for that, I'm going to pretend like I know what "intentional resistance values" are.

I found a 175 Amp breaker. that I will tie into the system. My intention is to run power from the trunk to a power distribution bar up front. then connect my starter wire, engine harness power and fuse box power to that.

For the ground in the trunk, Does anyone know where they connected it to from factory in the m5? Is there an ideal location?

:roll:
Galahad
Posts: 372
Joined: Sep 06, 2016 9:08 PM
Location: Beverly/Worcester, MA
Contact:

Re: Battery Relocation Wire Size

Post by Galahad »

Dirtym30 wrote: Feb 07, 2023 9:30 AM Thanks for that, I'm going to pretend like I know what "intentional resistance values" are.

I found a 175 Amp breaker. that I will tie into the system. My intention is to run power from the trunk to a power distribution bar up front. then connect my starter wire, engine harness power and fuse box power to that.

For the ground in the trunk, Does anyone know where they connected it to from factory in the m5? Is there an ideal location?

:roll:
Intentional resistance values: as opposed to unintentional ones (shorts). The starter is low enough resistance that it looks kinda like a short, it can be hard to automatically tell the difference between the two.

175 should be fine, power distribution bars are good. I don't know where the ground point is, maybe check the ETM?
iamcreepingdeath
Posts: 398
Joined: Dec 28, 2008 10:03 PM
Location: Mayer, MN

Re: Battery Relocation Wire Size

Post by iamcreepingdeath »

I am also planning to relocate battery to the trunk, and was considering wire size as well. I can buy huge wire at the local Fleet farm, but it is expensive and heavy and probably unnecessarily large.

My assumption is the wire size i will need ought to be the same size as the power cable going to the starter itself. I don't know what gauge it is, but it's not that big. Also, a good ground is needed, but a ground cable can be short and can be grounded to the car body right near the battery.
Galahad
Posts: 372
Joined: Sep 06, 2016 9:08 PM
Location: Beverly/Worcester, MA
Contact:

Re: Battery Relocation Wire Size

Post by Galahad »

iamcreepingdeath wrote: Feb 16, 2023 10:41 AM I am also planning to relocate battery to the trunk, and was considering wire size as well. I can buy huge wire at the local Fleet farm, but it is expensive and heavy and probably unnecessarily large.

My assumption is the wire size i will need ought to be the same size as the power cable going to the starter itself. I don't know what gauge it is, but it's not that big. Also, a good ground is needed, but a ground cable can be short and can be grounded to the car body right near the battery.
You're on the right track, but I have some input.
1: resistance is proportional to length, so using the same size as the starter will reduce power to the starter since there's higher resistance in the cables. It would probably work but ideally you'd get bigger wire. I don't know what size BMW used for the starter cable but I doubt they used something really overkill.
2: grounding to the car body near the battery isn't a great idea - steel is pretty high resistance and the car body is rather thin (even if you ignore potential rust problems). I know the M5s had a short cable to the frame(?) but if you're going to the trouble of making your own set I'd suggest making a ground as well.
iamcreepingdeath
Posts: 398
Joined: Dec 28, 2008 10:03 PM
Location: Mayer, MN

Re: Battery Relocation Wire Size

Post by iamcreepingdeath »

Galahad wrote: Feb 16, 2023 11:17 AM You're on the right track, but I have some input.
1: resistance is proportional to length, so using the same size as the starter will reduce power to the starter since there's higher resistance in the cables. It would probably work but ideally you'd get bigger wire. I don't know what size BMW used for the starter cable but I doubt they used something really overkill.
2: grounding to the car body near the battery isn't a great idea - steel is pretty high resistance and the car body is rather thin (even if you ignore potential rust problems). I know the M5s had a short cable to the frame(?) but if you're going to the trouble of making your own set I'd suggest making a ground as well.
All good points. If I am running a power cable from the trunk up to the engine bay, it wouldn't be that much more added effort to also run a ground cable along side it. And may as well bump up a wire size over the starter cable.
Post Reply